1. Field of The Invention
The invention relates to the field of noise reduction, and in particular to a noise reduction method and filter for implementing the method having particular usefulness in telephone communications systems.
2. Background Information
In telephone communications systems, noise can originate with various sources. Background acoustical noise represents one of the major impairments in telephone voice communications, especially in hands-free mobile telephone systems.
Over the years, many contributions to a solution for the problem of noise reduction for noise-corrupted voice signals in telephone communications have been made. One of the possible approaches is so-called "noise suppression," wherein the noise spectrum is estimated during pauses in the voice signal, and such estimates are used during voice containing periods following the pauses to reduce the noise content of the noise-corrupted information signal.
Such problems become more serious in high-noise environments, e.g., the inside of a car. A recent proposal on this matter is contained in an article by J. Yang, titled "Frequency Domain Noise Suppression Approaches in Mobile Telephone Systems", published in Proc. ICASSP, vol. 2, pp. 363-366, April 1993, hereby incorporated by reference. This article describes a further processing of the technique proposed by R. J. McAulay, M. L. Malpass in "Speech Enhancement Using a Soft-Decision Noise Suppression Filter", IEEE Transactions on ASSP, vol. 28, No. 2, pp 137-145, April 1980, hereby incorporated by reference.
In the previous articles a noise suppression method based on a modified maximum likelihood estimate is developed. Noise suppression is carried out by first decomposing the corrupted speech signal into different frequency subbands. The noise power of each subband is the estimated during non-voice periods. Noise suppression is achieved through the use of suppression factor corresponding to the temporal signal power over estimated noise power ratio of each subband.